MadSci Network: Earth Sciences

Re: Hardness of rocks

Area: Earth Sciences
Posted By: Steven Schimmrich, Grad student Geology
Date: Fri Oct 18 17:02:49 1996
Message ID: 844972915.Es

Hello Carolyn!

In reading your questions, I wanted to make sure you knew the difference between a rock and mineral because they are different things.

A mineral is a naturally-occurring, inorganic (not alive) solid. Minerals have a definite chemical composition. For example, the mineral halite, or table salt, is composed of sodium atoms (Na) and chlorine atoms (Cl) bonded together (NaCl is its formula). Minerals are also crystalline.

Rocks, on the other hand, are composed of one or more minerals. For example, the rock type limestone is composed of the mineral calcite while the rock type granite may be composed of the minerals quartz, feldspar, and biotite.

What is the hardest rock on Earth?

Well, the hardest mineral on earth is a diamond. For rocks, which are made up of one or more minerals, the hardest are those which contain quartz. There is no single "hardest rock" but some of the rocks that would be very hard and strong and well-cemented quartz sandstones (a sedimentary rock), quartzite (a metamorphic rock formed from quartz sandstone) and granites or rhyolites (both igneous rocks with a lot of quartz).

How many types and subtypes of rocks have been classified for planet Earth?

For minerals, almost 3,500 species are known. For rocks, that's harder to say. When I teach introductory students about rocks, I teach them the names of 7 major igneous rock types (granite, rhyolite, diorite, andesite, gabbro, basalt, & peridotite), 8 major sedimentary rock types (conglomerate, breccia, sandstone, arkose, siltstone, shale, limestone, & dolostone) and 6 major types of metamorphic rocks (quartzite, marble, slate, phyllite, schist, & gneiss).

Geologists who study rocks, called petrologists, have hundreds of names for different types of rocks. There were, for example, a little over 1,500 names for igneous rocks recognized by the International Union Geological Sciences in 1989.

Many rocks, however, are named by placing the names of abundant minerals in the rocks as a prefix. For example, a schist with a lot of biotite might be called a biotite schist. There are also many specialized names for common rocks such as limestone.

What is the most expensive rock in the world?

I don't know. Rocks aren't normally very expensive unless they contain some rare minerals or gems. Some building stones or rocks used for sculpture, for example Italian marble, may be fairly expensive but I really don't know much more than that.

What is the largest diamond in the world? What is its size and what is it called? Where is it?

The largest cut diamond is the Cullinan 1 at 530.20 carats. The diamond was cut into a pear shape and is in a scepter that is a part of the British Crown Jewels. When the diamond was found, just laying on the ground in 1905 in South Africa, it was 3,106 carats (over 1.25 lbs) and was cut into 105 gemstones.

The largest diamond found in the U.S. was called the "Uncle Sam" and was 40.23 carats and found in Murfreesboro, Arkansas in 1924.

Where is the biggest golden nugget located?

A couple of the largest nuggets found were the "Carson Hill" nugget in California in 1854 which weighed 88 kg (~175 lbs) and the "Welcome Stranger" nugget found in Australia in 1869 which weighed 90 kg (~200 lbs).

Thank you for your attention to our questions.

I hope I answered your questions.

- Steve

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